Cellular networks are deployed in a variety of environments including less populated rural areas, moderately populated suburban areas, and densely populated urban areas. The characteristics of wireless channels tend to change from one environment to another. When a cellular base station employs an array of antennas, it can be constructed to use one or more of a variety of smart antenna techniques. The performance of smart antenna techniques is known to be dependent on a many factors, such as the array configuration (e.g., the number, relative location, polarization of the antennas of the array), the location of the base station with respect to surrounding scatterers (e.g., above or below), the distribution of scatterers in the environment, and the velocity of the mobile station (MS), which may also be interchangeably referred to as a “mobile,” “wireless communication terminal” user equipment (UE), and “terminal” among other terms. The number of scatterers, the angular spread of the channel, the Doppler spread, and the delay spread of the channel along with the base station array type also impact system performance.
The array configuration (e.g., spacing and polarization) of the base station and the smart antenna transmission strategy is generally optimized by cellular network designers for a particular environment. For example, if there is a relatively low angular spread in the channel, then a deployment of a uniform linear array with a low spacing (e.g., half lambda) may be a good choice since this type of array is better at steering a beam that encompasses most if not all scatterers resulting in the mobile station receiving a boosted signal even if the mobile station is moving at high speeds. Therefore, for rural or suburban channels having these characteristics, base stations could be deployed with uniform linear arrays to support beam-forming. In another example, if the angular spread is large then beam steering is less important since the beam would likely not encompass all of the rays. Under these circumstances it may be more important to increase the system capacity through MIMO techniques. In this case an array with wider spaced elements and possibly different polarizations is typically desired. Therefore, in urban scenarios, the base stations could be deployed with arrays of widely spaced cross polarized elements to support MIMO transmission. It is apparent that the base stations used to serve a geographic region do not necessarily employ the same array configuration or the same smart antenna transmission strategy. In a geographic area in which portions can be classified as rural and other portions as suburban, then the base stations in the urban locations could be deployed with two antennas for supporting open-loop MIMO transmission/open-loop spatial multiplexing, while the base stations in the suburban locations could be deployed with eight antennas for supporting closed-loop beam-forming.
The various smart antenna transmission strategies require specific optimizations within the signaling and physical layer formats to achieve optimum performance. A first example is the type of transmission used for the control, reference symbols (pilots) and/or data channels. The type of transmission might, for example, be dedicated to a particular mobile station because, for example, it is a beam-formed transmission, the transmission is power controlled etc. On the other hand, the transmission might be for more than one mobile station in which case the data or control is said to be broadcast. A broadcast strategy may be more suitable for antenna configurations having a small number of elements such as only one or two. In either case, the receiver of the transmission (e.g., mobile station in the case of a downlink transmission) will need to have pilot symbols in order to be able to obtain a channel estimate, which is then used to detect the data or control transmission. In the case of dedicated transmission, the pilots may be dedicated (e.g., beam-formed with the transmission) or be broadcast if the receiver (e.g., mobile station) knows or is informed of the beam-forming weights. For broadcast transmission, the pilots also tend to be broadcast where, for example, a separate pilot sequence is sent from each transmit antenna enabling the receiver to estimate the channel between each receive antenna and each transmit antenna. Dedicated transmission may still be used for a transmission to a single mobile station even if the pilot type is broadcast. In this case, knowledge of the broadcast channel between the transmit and receive antennas as well as the beam-forming weights enables the receiver (e.g., mobile station) to determine the beam-formed channel, which is then used to detect the data or control transmission.
In prior art cellular communications, the array type may change from cell to cell according to operator deployment choices and other factors, but unfortunately other mentioned system configurations such as the data and control channel format are fixed for all environments. Fixing the type of data or control transmission for all types of environments results in lower system capacity or available range in one environment over the other. Ideally, the type of data or control transmission could be tailored to the base station configuration with its associated smart antenna strategy that are both optimized for the particular environment.
Unfortunately, a heterogeneous deployment comprising of a variety of base station configurations poses further challenges. For example, a typical mobile station roaming from one environment to the next—be it rural, suburban, dense urban, or indoor—will receive service from the base stations having a variety of configurations. This mobile station would need to detect the base station configuration including data and pilot formats. Thus there is a need for a mechanism that enables communicating the type of data or control transmission as well as the type of pilot format depending on the environment and/or the deployment of a particular base station.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.